1. Field of the Invention
The present invention relates to a resist pattern calculation method and a calculation program storage medium.
2. Description of the Related Art
In a lithography process, a resist pattern is formed by an exposure process in which light from a reticle pattern is projected onto a resist by a projection optical system to expose the resist to light, and a development process in which the exposed resist is developed. A resist pattern having a desired shape is desirably transferred and formed on the wafer. However, due to, for example, the optical proximity effect (OPE) and low k1, a resist pattern having a desired shape cannot be transferred onto the wafer in practice. This is one factor which deteriorates the device characteristics. Therefore, to improve the device characteristics, it is necessary to calculate the shape of a resist pattern.
To calculate a resist pattern, Japanese Patent Laid-Open No. 08-148404 discloses a method of calculating a resist pattern from a light intensity distribution obtained by convoluting a light intensity distribution computed based on an optical image. In the technique described in Japanese Patent Laid-Open No. 08-148404, the light intensity distribution of the optical image is convoluted using various variance values (to be referred to as diffusion lengths hereinafter) to obtain a diffusion length close to that obtained in the exposure result. In the technique described in Japanese Patent Laid-Open No. 08-148404, a diffusion length obtained in the modeling result is then applied to calculation of the convolution integral of the light intensity distribution of a pattern to be calculated, thereby calculating the size of a resist pattern.
Unfortunately, it is difficult for the prior art technique described in Japanese Patent Laid-Open No. 08-148404 to calculate the size of a resist pattern with high accuracy. The inventor of the present invention examined the cause of this difficulty, and concluded that with the current tendency toward low k1, a method of mathematically convoluting a light intensity distribution computed based on an optical image cannot represent the diffusion phenomenon of an acid in a resist with sufficiently high precision. This is for the following reason. FIG. 1 is a schematic view of a chemical reaction produced during exposure in a chemically amplified resist. During exposure, in a region with a high light intensity (bright region), an acid (H−) in the resist sequentially undergoes a chain reaction with a resist polymer. In contrast to this, in a region with a low light intensity (dark region), the acid (H+) is neutralized with a base (OH−) contained in a quencher. That is, the behavior of the acid differs depending on the magnitude of the intensity of exposure light (whether the region of interest is bright or dark). The prior art technique described in Japanese Patent Laid-Open No. 08-148404, in which the optical image is convoluted in the same mathematical form regardless of the magnitude of the light intensity, does not take into consideration the above-mentioned behavior of the acid.